1. Field of the Invention
The present invention relates to an optical transmission system for high frequency digital signals, and more particularly to such a system in which a laser diode transmitter is controlled by a relatively low frequency control signal having a small amplitude, and in which a receiver contains a light-sensitive component with a following amplifier and a signal processing circuit.
2. Description of the Prior Art
Remotely-fed intermediate stations are inserted at certain spacings in a transmission link into transmission systems for digital signals so that an amplitude and timewise regeneration of the digital signals occurs. Given the use of coaxial cables, the possibility exists of additionally transmitting the remote feed current over the communication cable and, moreover, additional signals such as, for example, signals of a service channel or of an alarm and report channel. Given optical transmission systems, a possibility exists on a case-by-case basis of transmitting such additional signals over an additional remote feed line. In view of a local feed of regenerators for the optical transmission system, however, the case can occur that no remote feed line is provided parallel to the overall optical transmission link. In this case, there is a need for an additional signal path which also proceeds over the optical transmission system and which is of low frequency in comparison to the digital signals.
An optical transmission system is known from "Electronic Letters", July 31, 1980, pp. 624-626, in which an additional channel for the transmission of monitoring information and as a service channel is provided. The transmission of the additional information thereby occurs by direct modulation of the bit rate of the transmitted data signals. The modulation causes an increase of the jitter, i.e. of the phase modulation of the digital signals, which, in conjunction with further disruptive influences under certain conditions, can lead to disruptions in the transmission of the digital signals.
A method for regulating the preconduction current supplied to the laser diodes is known from the German published application No. 28 41 433 in which a periodic oscillation having a relatively low frequency and a low amplitude in comparison to the transmission signal is superposed on the preconduction current. A portion of the light emitted by the laser diode is branched off, the periodic oscillation is regained and, after conversion and comparison to a reference signal, is employed for the regulation of the preconduction current.
A regulation of the modulation current of laser diodes corresponding to the preconduction regulation is known from the German published application No. 28 47 182, which corresponds to U.S. Pat. No. 4,292,606 which is fully incorporated herein by this reference.
A further circuit arrangement for the regulation of the laser threshold is known from the publication "Electronic Letters", Nov. 23, 1978, pp. 775-776, in which at least one small amplitude, low frequency signal is likewise superposed on the laser preconduction current and on the data signal to be transmitted. In this case as well, a portion of the light emitted by the laser diode is picked up by a photo diode, the low frequency signal is regained and, after corresponding conversion is employed for regulating the laser diode preconduction current and for regulating the modulation current. A transmission of the regulation signals over the optical transmission system is not considered in the prior art, particularly in view of the very low amplitude of the regulation signal.